the 'menu' method in class MultilinkHTMLProperty
contains an error:

at line 1144 you declare 'value = self._value',
which is a list.

at line 1160 you make 'value' an iterator over
'self._value'.

at line 1170 you call 'if optionid in value...'

the error I see is due to 'value' not being a list
in line 1170, which is clearly caused by line 1160.
As I guess line 1160's value variable is totally unrelated,
I renamed it to 'for v in self._value...' and everything
works again.